New kind of black silicon shows improved light-trapping properties


Scientists at the Institute of Automation and Control Processes in Russia have developed a novel kind of black silicon that they say has a higher near-infrared (NIR) light absorption and is better adjusted to absorb solar irradiance in the 1100-1800 nm range.

“We simply added one another absorbing layer atop black silicon, which replicates its morphology to some extent,” the research's corresponding author, Alexander Shevlyagin, told pv magazine. “We have already demonstrated solar cell operation of a silicon-silicide heterojunction without surface texturing in our previous work.”

In the paper “Mg2Si is the new black: Introducing a black silicide with >95% average absorption at 200–1800 nm wavelengths,” published in Applied Surface Science, the Russian research team explained that the new absorber layer was made of magnesium silicide (Mg2Si), which is an inorganic compound consisting of magnesium and silicon with an energy bandgap 1.5 eV. “Mg2Si is a Si-compatible optical material with superior absorbance from the ultraviolet (UV) to NIR regions compared to bare silicon,” it explained. “It has already demonstrated photovoltaic perspectives as homo-/heterojunction photodetectors and solar cells.”

The researchers implemented what they called the “silicidation” of the black silicon through a low temperature and scalable process that doesn't require an ultra-high vacuum and can be conducted by thermal evaporation and magnetron sputtering. They fabricated the black silicon substrate for black magnesium silicide preparation via reactive ion etching of a crystalline silicon wafer and then used used vacuum evaporation to cover the substrate surface with the Mg2Si layer.

“The reflectance spectrum of the Mg2Si covered b-Si nanocones demonstrates strong NIR antireflection performance when compared to starting b-Si surface,” the academics emphasized. “Our measurements confirm averaged reflection in the 200-1800 nm spectral range of 3.7% from black magnesium silicide surface, which is 5 times lower with respect to 17.6% of the bare black silicon.”

According to them, the material's improved light trapping properties should be attributed to the complex hierarchical nanocone-nanoflake
structure realized in the new compound, which resulted in the marked increase of both NIR antireflection and absorption properties. “NIR photons after being back-scattered within b-Si nanocones are effectively absorbed by Mg2Si possessing much higher intrinsic absorption coefficient over the wide spectral range,” they further explained.

The group called the new wide-band black material “black silicide” and said it may also be used for NIR optical absorbers and highly sensitive NIR devices. “The results obtained propose a simple technology to further enhanthe photoelectric conversion efficiency of the silicon-silicide tandem solar cells using a black silicon instead of pyramid-like Si surface textures as a platform for silicidation with other environment-friendly and silicon compatible silicides,” they concluded.

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